core: improve block rate monitor trigger probabilities

The original intent of one false positive a week on average
was not met, since what we really want is not the probability
of having N blocks in T seconds, but either N blocks of fewer
in T seconds, or N blocks or more in T seconds.

Some of this could be cached since it calculates the same fairly
complex floating point values, but it seems pretty fast already.

src/cryptonote_core/cryptonote_core.cpp

@@ -1784,12 +1784,28 @@ namespace cryptonote
     return f;
   }
   //-----------------------------------------------------------------------------------------------
-  static double probability(unsigned int blocks, unsigned int expected)
+  static double probability1(unsigned int blocks, unsigned int expected)
   {
     // https://www.umass.edu/wsp/resources/poisson/#computing
     return pow(expected, blocks) / (factorial(blocks) * exp(expected));
   }
   //-----------------------------------------------------------------------------------------------
+  static double probability(unsigned int blocks, unsigned int expected)
+  {
+    double p = 0.0;
+    if (blocks <= expected)
+    {
+      for (unsigned int b = 0; b <= blocks; ++b)
+        p += probability1(b, expected);
+    }
+    else if (blocks > expected)
+    {
+      for (unsigned int b = blocks; b <= expected * 3 /* close enough */; ++b)
+        p += probability1(b, expected);
+    }
+    return p;
+  }
+  //-----------------------------------------------------------------------------------------------
   bool core::check_block_rate()
   {
     if (m_offline || m_target_blockchain_height > get_current_blockchain_height())